Radio paging systems have been in use for many decades for sending page messages to radio paging units, such as pagers. Historically, radio paging systems have been one-way communication systems which broadcast radio paging messages to radio paging units, often worn personally by their users. Each message so broadcast includes certain coded information which enables a select radio paging unit (or a select group of radio paging units) to receive the message. In response to a received message, the radio paging unit usually alerts the user (audibly, visually or tactilely) so that the received message (numeric, alphanumeric or audio) might be retrieved by the user.
In the earliest radio paging systems, radio paging messages were broadcast from a single transmitter site. A site has an encoder for encoding page message, and a controller which sends such encoded messages via a transmitter and antenna. Due to the finite geographic coverage that can be realized between a transmitter and a receiver, radio paging systems now include the broadcasting of radio paging messages from one or more geographically dispersed transmitter sites, which may be grouped into transmission systems. Increasing the number of geographically dispersed transmitter sites broadens the geographic coverage of radio paging message delivery, thus increasing the user's freedom of mobility, and the convenience value of the paging service to the user. Increasing the number of transmitter sites, however, increases the cost of service, thus decreasing the attractiveness of the service to the user. The cost of service may be defined as a function of the number of transmitter sites used in the system divided by the radio paging message capacity that can be carried by the system. As each transmitter site has finite information carrying capacity, limiting the cost of service lies in limiting the number of transmitters used to deliver each radio paging message. Limiting the number of transmitter sites used to deliver a message without restricting the user's mobility requires that the system be able to broadcast messages only from the transmission site(s) actually required to deliver a message to a given radio paging unit or group of radio paging units, wherever the radio paging unit of the group might be at that moment in time. However, this requires the system to have knowledge regarding the location of the radio paging unit when messages are to be delivered.
Generally, there are three types of radio paging systems which propose solutions to the problem of determining the location of a radio paging unit. One type of radio paging system provides for the user to place a local telephone call to a local paging destination controller whenever the user arrives in a new area. The user identifies himself or herself and identifies his or her current area as that of the destination controller to which the phone call is made. This destination controller then contacts another controller which is associated with the origination of radio paging messages for the user and informs this controller that it is now the destination controller to be used for broadcasting radio paging messages to that user. Thereafter, until new information is input into a destination controller, radio paging messages are routed from the origination controller through land lines to the destination controller where the user's radio paging messages are broadcast from an associated transmitter.
There are a number of drawbacks associated with this type of system. The telephone number for the destination controller that the user must call is not usually a number that the user would readily know. Further, users traveling nationwide would be required to know thousands of telephone numbers for thousands of destination controllers, since each destination controller controls one transmitter site. The limited area of coverage from one transmitter site may require the user to make frequent telephone calls to different destination controllers. In addition to accessing the destination controllers, the users have no knowledge of when they are leaving an area covered by one destination controller (a transmitter site) and entering an area covered by another destination controller (another transmitter site), hence the user is unaware of when he or she needs to contact a new destination controller.
A second type of radio paging system also provides for the user to place a telephone call, but only to one known destination controller whenever the user arrives in a new area. In this type of system, the radio paging unit has means for determining its geographic position, such as via a Global Positioning System (GPS). When the user calls the destination controller, the user's identification information and geographic position information are communicated to the destination controller which, in turn, forwards the information to an origination controller associated with the user. When messages are received at the origination controller for delivery to the subject radio paging unit, the origination controller uses the geographic position information it has received to determine the appropriate delivery controller to route the message to so that the message can be broadcast from the transmitter site(s) serving the location defined by that geographic position information.
As with the first type system, there are several drawbacks associated with this second type system. This system requires the user to place long distance telephone calls back to the one destination controller assigned for use by this user in order to update his or her current geographic location information. The burden still rests with the user to determine when his or her geographic location has changed significantly enough to require a long distance telephone call to update geographic location information. This may be difficult since the user generally is unaware when he or she is leaving an area covered by one transmitter site and entering an area covered by another transmitter site. Further, the paging unit would be expensive since it may require additional circuitry for providing the geographic location information.
Both of the first and second types of systems provide for locating and controlling page message transmissions at a transmitter site level. This will often require frequent location updates to the system due to the limited coverage of a transmitter site as radio paging units move in large metropolitan areas, or when radio paging units are worn on users traveling through many geographic regions in a short period of time, such as on interstate highways. For example, if the coverage area of a transmission site is a 10-mile radius, a user traveling in a car at 60 miles per hour could require location updates every 20 minutes.
A third type of radio paging system provides for a radio paging unit with an acknowledgment transmitter to send a signal back to the home paging terminal when the radio paging unit arrives in a new location. The radio paging system transmits location identifiers, which, for example, may represent, the city, a service area, or geographic region. The radio paging unit determines that it has arrived in a new location when it receives a location identifier different from its home identifier. When the home paging terminal is notified of the radio paging unit's new location, the system forwards messages to the new location.
There are several drawbacks with this third type of radio paging system. The system requires the radio paging system to provide two-way radio paging, in which a companion frequency is provided for the transmission and reception of acknowledgment transmissions by the radio paging unit via the unit's acknowledgment transmitter. This requires a comprehensive network of receivers to receive the low power transmissions from the radio paging units in all locations where message delivery is required. Such receiver networks are costly to construct and maintain, resulting in higher paging service costs. Further, the system can operate only over limited geographic service where receiver networks are provided, and the incorporation of a radio transmitter in the radio paging unit to provide two-way radio paging increases considerably the cost of the radio paging unit.
The third type of radio paging system may use a cordless telephone, rather than an acknowledgment transmitter, to call the home paging terminal when arriving in a new location. This however requires the availability of cordless telephone service in all locations where message delivery is required. A situation not usually controlled by a radio paging service provider. Thus, universal service, geographically, cannot be assured. While potentially more convenient to users than the previous two types of radio paging systems, this system depends on an expensive receiver network or, in the alternative, the placement of long distance telephone calls to communicate with the home paging terminal in order to reroute message delivery to the current location. An example of the third type of radio paging system is described in U.S. Pat. No. 5,684,859.
Another aspect of radio paging is assuring message delivery when messages are routed to particular transmitter sites. As a practical matter, when users move about in wide area radio paging systems, there will be periods of time when messages are undeliverable due to limitations in radio coverage of the transmission site through which page messages are being sent. For example, when a user moves from one location in the system to another, there may be periods of time when the user's current location is not covered by any transmitter site. Further, there will always be delays between the user's actual movement to a new location and the updating of such location change information to the system. It is unlikely that messages arriving during these transitional periods will be delivered to the users. Without assuring page message delivery, messages may be lost if the user fails to update his or her location to the paging system as described in the above types of radio paging systems.
To assure message delivery in the first type of radio paging system discussed above, the user is required to place a local telephone call to the current destination controller before leaving the area to cause the system to hold his or her messages until he or she arrives at the new area, and then a second local telephone call to the associated destination controller to cause stored and future messages to be delivered to the new destination controller. This is inconvenient to users, especially when they may be unaware when they have left or entered a new area. The second and third types of radio paging systems described above lack a mechanism for assuring message delivery to paging units.